Underfloor ventilation conundrum

[JP_]

OK, so can you not lower the ground level either side of the extension to allow for the ventilation, and just have the rear side level with ground?
Or, continue the ventilation under the patio and have a step down to lawn, or whereever, or just a trench, where the vents can come out?

 

[vexation]

OK, so can you not lower the ground level either side of the extension to allow for the ventilation, and just have the rear side level with ground?

That's the exact conclusion we've drawn here right now. Rather than level thresholds all round we'll construct a level patio exclusively off of the main elevation that we plan to make most use of and have a drop to the sides. One side was to be lawn anyway and the other side can be stepped down with a low retaining wall to define a separate paved area.

Or, continue the ventilation under the patio and have a step down to lawn, or whereever, or just a trench, where the vents can come out?

Unfortunately the lawn just keeps on sloping up the further out we go so there's no natural place to extend a vent out to. Trenches would also have to be drained and just defeat the purpose of a level threshold. Perhaps some enterprising drainage manufacturer could make a combi drain/vent with the invert set below a vertical grating for mating to a telescopic vent on one side.

It seems that we are blazing a trail wanting a level threshold alongside floor to ceiling glazing after all.

 

[JP_]

blazing a trail wanting a level threshold alongside floor to ceiling glazing after all

Probably with a suspended floor, but maybe this is why concrete slabs are often the best option. But a bit late now! Sounds like you have a solution so no worries.

 

[Leofric]

The oversite concrete should be above ground level on a level site to prevent water collecting in the underfloor void. On a sloping site the top of oversite concrete should still be above the lowest adjacent ground level . Wouldn't it be best if your architect now drew a cross section/s through the floor showing adjoining ground levels and methods of ventilating the sub floor before you go any further on site. You are obviously now aware of the problem with level thresholds with suspended precast concrete beam and block (or timber ) ground floor construction ,complicated by a sloping site. You can't have vents coming out at ground level where water could run down the vent and collect on the oversite concrete. I am not convinced you have found a solution yet. Sometimes certain forms of construction are just not compatible and floor to ceiling glazing with suspended ground floor on a sloping site might be one of those cases.

 

[tomfe]

Sticks a solid concrete floor in and be done with it, it may cost you a couple of extra Ks now but it will give you peace of mind and probably save you money on hard scaping later.
Having a concrete pond at the bottom of a hill is asking for problems.

If your levels are drastic use EPS to keep back fill small.

Remember all that water will run down towards your house so make sure you have adequate drainage.

 

[vexation]

Top of vents on two sides are 150 above ground so they should be no problem. Perforated pipe going around at foundation level to drain off underfloor void should groundwater make its way through the clay. Any other precautions come to mind? Solid slab has been considered but the volume of concrete required would be eye-watering. The foundations are already well over 1m deep due to trees and have swallowed a large chunk of the costs.

Incredibly we're not at the bottom of a hill, far from it. 23m above the local river level and that's more than 100m away.